THE EARLY BODY FOSSIL RECORD OF EUKARYOTES

Paleoproterozoic and lower Mesoproterozoic rocks preserve a number of fossils that exhibit character combinations known today only in eukaryotes, e.g., large cells with ornamented, acid-resistant walls, or complex macroscopic forms. These fossils have been tentatively interpreted as stem-group eukaryotes in part because they lack characters diagnostic of specific crown-group clades. Fossils that are more confidently interpreted as crown-group eukaryotes occur in upper Mesoproterozoic and Neoproterozoic rocks. These include Bangiomorpha and Palaeovaucheria, suggested to be red and xanthophyte algae, respectively; ‘Tappania’, thought to be a fungus; and vase-shaped microfossils, interpreted as arcellinid and euglyphid amoebae.

There are difficulties with these interpretations, however. The first stems from molecular phylogenetic evidence suggesting that the last common ancestor of living eukaryotes may have been a small (5-10 µm) single-celled flagellate without preservable structures. In other words, the characters used to identify the ‘stem-group’ fossils as eukaryotes in the first place—large cells with ornamented, acid-resistant walls or complex macroscopic forms—are not plesiomorphic to crown-group eukaryotes but rather evolved within the crown group many times. Thus, if these fossils do represent stem-group eukaryotes, they must have evolved these characters convergently with the living eukaryotes. A second difficulty with interpreting early eukaryotic fossils comes from evidence for widespread convergent evolution among living eukaryotes, including microbial forms. Testate amoebae evolved several times, for example, and the characters that link Palaeovaucheria to xanthophyte algae are also found in water molds and siphonous green algae. Despite questions about their taxonomic affinities, however, these fossils do provide information about biological innovation and diversification through the Proterozoic.